Ion-sensitive electrodes measure the activity of ions in solution (both aqueous and non-aqueous)and are well known in the art of analytical chemistry. One example of such a measurement is pH, which is a measure of the activity of hydrogen ions in solution, and is an important parameter for many chemical processes. Another example is the measurement of sodium ions in foods or biological fluids.
Ion-sensitive electrodes are commonly formed from a tubular shell having one end sealed with an ion-selective membrane. The membrane is selectively permeable to ions of one type, while excluding others present in the sample solution. Inside the tube there is a means for providing a fixed potential, either a solution of fixed composition or a solid conductor in contact with the membrane. The potential across the membrane, measured from the internal contact, through the sample to a second reference contact provides a measure of the sample ion activity.
Ion-selective membranes are most commonly formed with either a bulbous or a flat shape. For membranes formed in the glassy state, bulbous-shaped electrodes are more readily formed than flat-membrane electrodes, and are suitable for measurements of liquid samples where there is a significant quantity of liquid available for measurement. Flat-membrane electrodes, in contrast, are desirable, or even required, for measuring samples where there is a limited quantity of material available, and for measuring moist solids where the membrane must be pressed against the sample without immersion in it.
The membranes used for ion-sensitive electrodes typically present a high input impedence to the measuring instrument, e.g., up to 1000-20000 megohms. This impedence limits the accuracy of measurements because of noise pickup in the electrode. In particular, the ion-selective membranes for pH-sensing electrodes are commonly formed from glass. In common pH-sensing glasses, high selectivity for a hydrogen ion is typically also accompanied by high resistivity, and thus the improved sensitivity otherwise obtainable from the material is masked by the increased noise pickup caused by the higher resistivity. This can be particularly a problem with flat surface membranes in which conventional manufacturing techniques place stringent limits on the extent to which the membrane thickness (and thus, its resistance for a material of given resistivity) may be controlled.
Flat-membrance surface ion-sensitive electrodes are commonly constructed by a dipping process in which a tubular section of glass is immersed in a molten bath of membrane material. A bead of molten material typically adheres to the end of the tubular section, and is fabricated into a flat membrane on cooling. The molten glass must have a coefficient of expansion closely matching that of the tube. If the coefficients of expansion of the tube and the molten glass differ greatly, either the tube or the membrane material will frequently crack upon cooling, due to differing rates of contraction. Further, the seal between the tube and the membrane glass is often irregularly formed and prone to failure. In addition, dipping processes are difficult to control for uniformity and repeatability of membrane thickness. Sample to sample thickness variations may lead to large variations in strength or electrical resistance.
Once the dipped tube has cooled, the pH glass may be ground to a desired thickness for the flat membrane required. Grinding is a time consuming process and results in a high percentage of defective electrode bodies due to accidental breaking of the thinned membrane material. Further the grinding process introduces microgrooves and stresses into the membrane. Impurities from the grinding material may also embed themselves into the areas that are ground and thereby distort membrane properties. Finally, there is a physical limit to the thickness to which one can grind a material, without breaking that material. The limitation is due to the impact nature of the grinding process and the brittle nature of membrane material. This limitation has prevented the use, in flat or substantially flat membranes, of low-sodium interference high-resistivity glass.
A need therefore exists for a new method of manufacturing electrode bodies which will allow for the development of improved electrodes utilizing improved materials and having none of the drawbacks of conventional electrodes.